The present invention relates to an electrodeless discharge lamp in which an induction coil is arranged in a cavity provided in a bulb, in particular, an electrodeless discharge lamp having a thermal conductive member.
Conventionally, electrodeless discharge lamps using inductive coupling plasma have been used for illumination of public facilities such as roads or bridges for the purpose of reducing the maintenance cost, because they have a long lifetime. However, in a recent trend, the electrodeless discharge lamps are increasingly used as a light source alternative to incandescent lamps in hotels or restaurants because they have high efficiency and long lifetime. In the development of the electrodeless discharge lamps, efforts are put to achieve a lamp having good start-up properties and a high efficiency, that is, to supply power to a discharge bulb from a commercial power source via ballast circuits as efficiently as possible.
Conventionally, in order to supply electromagnetic energy to the discharge bulb of an electrodeless discharge lamp efficiently, it is general to attempt to achieve impedance matching between an inverter circuit and a load resonance circuit (matching circuit) included in the ballast circuit so as to supply the maximum power to the induction coil. In this case, the electromagnetic energy supplied to the discharge bulb via the induction coil is affected significantly by the inductance of the induction coil included in the load resonance circuit. That is to say, if the inductance of the induction coil is even only slightly outside of the designed value (e.g., 2 to 3%), the resonance frequency of the load resonance circuit is not matched to the operating frequency of the inverter circuit (driving frequency of the switching element). Thus, if the two frequencies are unmatched even slightly, the resonance voltage applied across the induction coil is reduced significantly, so that the electrodeless discharge lamp cannot be started.
For this reason, it is desirable that the impedance element constituting the load resonance circuit has no tolerance in the characteristics so that the resonance frequency can be constant. In this background, Japanese Laid-Open Patent Publication No. 10-69992 discloses a movable cylinder for fine tuning of coil inductance for the purpose of fine tuning of the tolerance in the impedance of the inductance coil.
Furthermore, the operation and the efficiency of the electrodeless discharge lamp are affected by the temperature characteristics of ferrite that is a magnetic material used as the core of the inductance coil. When the temperature of the core is increased by the heat generated in the core of the induction coil, the magnetic permeability of the core is reduced. An electrodeless discharge lamp in which a thermal conductive member that dissipates the heat generated in the core efficiently is provided in order to prevent the reduction of the magnetic permeability due to this temperature increase is put into practice. For example, Japanese Utility Model No. 6-6448 discloses an electrodeless discharge lamp in which a rod-shaped thermal conductive member is provided along the principal portion in the length of a cylindrical core. This publication No. 6-6448 also discloses a structure in which the heat of the core transmitted to the rod-shaped or cylindrical thermal conductive member is transmitted to a case via a plane-shaped thermal conductive member provided perpendicularly to the core and is released to the outside of the case.
Japanese Patent Publication No. 5-27945 discloses an electrodeless discharge lamp in which a cylindrical thermal conductive member is provided along the inside of the core in order to effectively dissipate the heat generated in the inductance coil, and the thermal conductive member is electrically insulated from the metal housing including a power source unit to reduce the start-up voltage.
In order to ensure the start-up of the electrodeless discharge lamp, it is necessary to make the supply power to the discharge bulb as much as possible. For this, it is important to suppress tolerance in the inductance of the induction coil. This has been referred to in the description of prior art. Furthermore, the inductance of the induction coil is also affected by the arrangement relationship between the thermal conductive member provided in the electrodeless discharge lamp to dissipate the heat and the induction coil.
However, there has been no report that proposes specifically what to do in order to suppress the tolerance of the inductance that is generated by the arrangement relationship between the thermal conductive member and the induction coil.